1. Field of the Invention
This invention relates to a composite tactile sensing device able to provide a robot with a set of sensory capabilities comparable to those provided by the human skin.
Present generations of robots lack most of the sensorial abilities of humans. This limitation prevents industrial robots from being used to carry on delicate tasks of enormous practical relevance (such as assembly operations) and, even more, it prevents the development of evoluted robots for off-factory jobs (agriculture, home, assistance to the disabled, etc.) It is not difficult to anticipate therefore that the next generations of robots will be increasingly featured by the massive use of dedicated sensors which will enhance substantially the limited ability of present robots to interact with the external world. Taction, vision and proximity are the sensory needs that, in combination or alone, are commonly accepted as desirable features of robots. Research on visual pattern recognition received considerable attention in recent years. Tactile sensing did not attract comparable research interest in the past; present state of the art in automated tactile sensing is recognized, in fact, as rather primitive.
Tactile recognition (the ability to recognize objects by manipulation) is an inherently active process. Unlike visual sensors (passive and located remotely from the object), tactile sensors must be put in contact with the object to be recognized and, even more, such contact should be competently organized in order to extract the maximum degree of information from manipulative acts.
Therefore, tactile sensing requires complex system architecture, including evolute tactile sensors, dexterous hands, suitable exploratory strategies, efficient control and advanced signal processing capabilities.
This invention relates to one important aspect of tactile sensing, i.e. the tactile sensor itself. Human tactile sensing undoubtedly represents a useful model for robots, also because the specific needs of robotic tactile sensing are still not well defined. Therefore, it may be convenient to assume the human skin as a model for artificial tactile sensors. An artificial skin sensor should possess, as the natural skin, softness, elasticity and some mechanical resistance. Furthermore, it should detect contact pressure, material hardness, surface texture and slippage. An additional desirable feature of the artificial tactile sensor should be the ability to identify different materials on the basis of their thermal properties.
Presently, no commercial robot is provided with evoluted tactile sensor. Only a few robot grippers possess very simple contact force measuring devices of the strain-gauge type. Most robots only have on-off, microswitch type transducers to assess the contact between the grippers and the object to be grasped. However, various models of tactile sensors have been proposed and developed by several research groups.
A number of different techniques and materials have been utilized for the construction of artificial skin-like sensors, including conductive rubbers, strain gauges, piezo-electric transducers, magnetic and capacitive sensors. See for example the article "A high-resolution image touch sensor", by W. Daniel Hillis, The International Journal of Robotics Research, vol. 1, No. 2, summer 1982, the article "Design and implementation of a VLSI tactile sensing computer" by Marc R. Raibert and John E. Tanner, The International Journal of Robotics Research, vol. 1, No. 3, fall 1982, the article "Conductive elastomers as sensor for industrial parts handling equipment" by Wesley E. Snyder and Joseph St. Clair, IEEE transactions on instrumentation and measurement, vol. IM-27, No. 1, March 1978, the chapter "Interactions involving physical contact between robot and environment: tactile detection", Robot Technology, vol. 2, by Philippe Coiffet, chapter 4, pages 75-89, the article "Torque-sensitive tactile array for robotics", by S. Hackwood et al, The International Journal of Robotics Research, vol. 2, No. 2, summer 1983, and the article "Capacitive impedance readout tactile image sensor", by R. A. Boie, Proceeding of the IEEE Conference on Robotics, March 1984.
Although generally able to provide some degree of information on tactile sensing, none of the previously proposed devices was entirely satisfactory and limitations in terms of robustness, reliability, cost, sensitivity, noise, hysteresis, non-linearity, fatigue, drift and long time constant have been reported. In a different field, an instrument for viscoelastic measurement (see U.S. Pat. No. 4,250,894) has been described, making use of an array of piezo-electric polymer sensors for viscoelastic measurement by palpation. However, such known apparatus does not provide an accurate measurement.